I am doing a practice unit plan for a 4th grade class about the importance of water. The first lesson plan in the unit should focus on a comprehension skill activity that involves writing. I went to the library and i choose the book "The Drop in my Drink" by Hooper and Coady. However, I have am having a difficult time finding the most appropriate comprehension activity to do with the book.... Do you have any ideas?

I want my readers to understand that water has been around from the beginning of time. I want them to understand that the earth is constantly recycling the water and that the waters that dinosaurs drank millions of years ago, is the same one that we are drinking now.
This is basically what the book talks about.... now I wanted them to do a written activity... maybe finding the main idea.... or something to that effect.